The present invention relates to permanent magnet electric motors of the radial air gap type. More particularly, the present invention relates to permanent magnet electric motors of the radial air gap type in which the axial alignment of the rotor and stator can be varied to control speed and applied torque.
Permanent magnet electric motors are known to be efficient producers/users of rotational power. Brushless permanent magnet motors eliminate the commutator and brushes that are commonly used with an electric controller that switches current to conductors in more conventional electric motors. The brushless electric controllers used in permanent magnet electric motors allow the conductor coils to be located in stationary motor stators, which react to movement of rotors containing permanent magnets.
Brushless permanent electrical motors are known for their durability, controllability, and absence of electrical sparking. These factors pose problems in commutator equipped electrical motors.
Unlike many other types of electrical motors, permanent magnet electrical motors have a linear relationship between the product of EMF (voltage) x current (amps) and output torque. This characteristic is ideal for the traction loads of electric vehicles and other applications where a linear torque/speed relationship simplifies control.
Conventional permanent magnet motors can apply high output torques up to an rpm limit called the base speed. The base speed rpm is governed by the phenomena of permanent magnet motors building up xe2x80x9cback-emfxe2x80x9d electrical potentials as rotational speeds increase. The back-emf is governed by the magnetic gap flux density, number of winding turns, and rotational speed. As the rotational speed of a permanent magnet motor increases, the back-emf will build up until it equals the supplied voltage. Once the back-emf equals the supplied voltage, permanent magnet motors will not operate any faster. This back-emf rpm limiting characteristic protects permanent magnet motors from the over speed damage that is common with series wound electrical motors used in vehicle applications. The back-emf base speed characteristic that protects permanent magnet motors also tends to limit the dynamic rpm range.
In order to accelerate from rest or from low speeds, many electric vehicles have a fixed reduction drive ratio that is set for high torque. While such configurations provide the necessary high torque to overcome inertia, it results in a low base speed and a limited top speed. In addition to a low speed, constant torque operation, it is desirable for many motor vehicles to also have an upper range of constant power, where speed can increase with decreased torque requirements.
There are methods by which to operate a brushless permanent magnet motor or other motor type beyond the base speed. These methods can be broadly classified as either those using electrical means or those using mechanical means.
Methods of electrically enhancing speed or varying magnet flux include high current switching of additional phase coils or switching the way the phase coils are connected. The costs of such contactors and their contact wear tend to negate the advantages of a high durability brushless motor. Supplemental flux weakening coils have also been used to reduce stator flux and increase speed. This latter approach typically requires contactors and increases heating effects in the stator. Other methods can achieve higher speed operation by varying the waveform shape and pulse angle of the applied driving current or voltage.
Other known methods include the use of DC/DC amplifier circuitry to boost the supply voltage in order to achieve a higher motor speed. This method increases system costs and decreases reliability and efficiency. Such electrical approaches to increasing a motor""s base speed are exemplified in U.S. Pat. Nos. 5,677,605 to Cambier et al., 5,739,664 to Deng et al. and 4,546,293 to Peterson et al.
Mechanical approaches to increasing a motor""s base speed include configurations that vary the radial air gap between a tapered or conical rotor and stator. U.S. Pat. Nos. 829,975 and 1,194,645 to Lincoln disclose a conical rotor and shaft that is moved axially by a worm gear to adjust air gap and speed. U.S. Pat. Nos. 3,648,090 to Voin and 4,920,295 to Holden et al. each disclose a conical rotor in an alternator that is adjusted axially to vary air gap and the alternator output. U.S. Pat. No. 5,627,419 to Miller discloses a conical rotor that is moved axially to increase air gap and reduce magnetic drag on a flywheel energy storage system when the motor is not energized. In all of these patents, the rotor and stator remain engaged and changes in the magnetic air gap is achieved by small axial movements.
U.S. Pat. No. 3,250,976 to McEntire discloses motor stator coils of an AC induction motor that are moved axially between shorted and non-shorted portions of a dual rotor to vary speed. McEntire requires complex multiple lead screws or ball screws to effect stator movement and a double length rotor.
U.S. Pat. No. 5,821,710 to Masuzawa et al. discloses a magnet rotor that is split into two sections. For normal slow speed operation, the magnetic north and south poles of both rotor sections are aligned. As motor speed increases, centrifugal weights rotate one rotor section so the magnetic poles have increasing misalignment with speed. The magnetic pole misalignment causes a reduction in magnetic flux and back-emf, which allows the motor to run faster than normal base speed. This system is self contained, but requires a split rotor and the centrifugal apparatus to move the one rotor segment into misalignment. The strong repulsive forces of like magnet poles produce thrust to push the rotor segments apart. When the poles are misaligned, the attractive forces of unlike magnetic poles add to the centrifugal positioning force and override the springs used to restore the alignment position. These factors add to the complicated design and effect durability, and cost.
U.S. Pat. No. 6,194,802 to Rao discloses a pancake type motor that uses a fixed axial air gap. In this type of motor the axial gap is functionally equivalent to the radial gap in an internal cylindrical rotor motor design with a radial air gap. The individual magnet sectors in the rotor are mounted on spring loaded radial tracks. When the rotor rpm increases, centrifugal force causes the magnet sectors to extend radially, reducing the active area of magnet aligned with the stator coil and reducing the back-emf. This causes the motor to run faster than the base speed. Rao is similar to Masuzawa et al. and Holden et al. mentioned above in the centrifugal method of activation. The design of Rao requires extensive machining of the radial magnet tracks which increases costs and adds to the complexity. In addition, maintaining a sufficient level of balance of this magnet rotor is complicated by several factors. Even after the rotor is balanced with the magnets at their inboard position, as speed increases the position of the individual magnets is affected by difference in mass of the magnets, spring constants/rates, and sliding friction of the magnets along the tracks. Small variations in the resultant in the individual magnet positions would have a disastrous effect on the balance at high rotor speeds. These factors would necessarily adversely effect the ability to reduce back-emf of the motor and operate above the base speed.
According to various features, characteristics and embodiments of the present invention which will become apparent as the description thereof proceeds, the present invention provides a brushless permanent electrical motor which includes:
a stator having a plurality of stator magnetic poles and windings for generating a rotating field in the stator magnetic poles, said stator having a central axis and a substantially uniform inner diameter;
a rotor provided with a plurality of permanent magnets at a peripheral surface thereof and having a central axis which coincides with the central axis of the stator, the rotor further having a substantially uniform outer diameter;
a rotatable shaft upon which the rotor is coupled, the rotatable having a central axis which coincides with the central axis of the stator; and
means for moving the rotor with respect to the stator along the rotatable shaft.
The present invention further provides a method of operating a brushless permanent magnet electric motor beyond its normal base speed which involves:
providing a brushless permanent magnet electric motor having a stator, a rotor, and a rotatable shaft upon which the rotor is coupled; and
moving the rotor with respect to the stator along the rotatable shaft.